There is a continuing need for new and improved alloys that are suitable for high temperature structural components and casing applications in gas turbine engines. The requirement for new alloys is driven by the desire to operate gas turbine engines at higher temperatures and pressures to achieve increased fuel efficiency. Benefits may also be derived from new alloys with increased microstructural stability during service, enabling increased component life. In particular, new alloys with higher strength may allow designers to reduce wall thickness and/or component weight, thus leading to the ultimate benefits of reduced material usage and greater efficiency. In addition, there is a drive for improved alloys that are amenable to welding, repair, or production through additive manufacturing techniques.
One class of existing alloys amenable to welding and additive manufacture is the group of alloys commonly referred to as nickel-based superalloys that contain comparatively low volume fractions of reinforcing precipitates. Examples of such known alloys include, for example, Inconel 718 (IN718), Inconel 725 (IN725) and René 220.
As known in the art, IN718, as disclosed in U.S. Pat. No. 3,046,108, was designed to precipitate a distribution of gamma double prime (γ″) precipitates along with a very small distribution of gamma prime (γ′). IN718 is known as a malleable nickel-chromium base alloy having a particularly high combination of strength, ductility and rupture strength at temperatures of up to 760° C. Consequently, developmental work on IN718 established a method by which the precipitates could be formed with a compact morphology consisting of a y cube with a layer of γ″ covering all sides of the outer faces. As such, IN718 is commonly processed to produce a microstructure in which the γ″ nucleate and grow from a fine dispersion of γ′ precipitates formed at a higher temperature. This leads to a sandwich like morphology in which the γ′ precipitates are enveloped by γ″. This modification was reported to confer improved mechanical properties, as disclosed in U.S. Pat. No. 3,972,752.
Additionally, U.S. Pat. No. 7,527,702 describes a Nickel-base alloy known as Allvac 718Plus. Allvac 718Plus is a predominantly γ′ strengthened alloy, which also precipitates a grain boundary phase: eta (η) (Ni3Ti) or delta (δ) (Ni3Nb). The Al is therefore the primary gamma prime forming element, but the Nb and Ti will also be present in the γ′ and help to strengthen this phase.
Furthermore, U.S. Pat. No. 4,788,036 describes a Nickel-base alloy known as IN725, containing correlated percentages of chromium, iron, molybdenum, titanium, niobium and aluminium. IN725 is strengthened by γ″ precipitates, with a small dispersion of γ′. This alloy was reported to possess good workability, high strength, ductility and resistance to both pitting and stress-corrosion cracking.
Accordingly, several other alloys, such as Ticolloy, have been developed to strike a balance between the γ′, γ″ and δ phases, tailoring them to meet the thermal, mechanical and microstructural stability requirements for varying industrial applications. In particular, Ticolloy is listed as having the same composition as IN718, but with a modified Al, Nb and Ti content [Tien et al., Proceedings of the 1990 High Ttemperature Materials for Power Engineering Conference, p1341-1356, 1990].
It is known that the ease with which superalloys may be fabricated, welded and thermo-mechanically processed decreases with increasing γ′ volume fraction as rapid cooling does not suppress precipitation. This leads to lower ductility and increased susceptibility to cracking during processing. Those alloys that derive their strength from γ″ have the benefit that the precipitates only form on slow cooling or subsequent heat treatment, thereby making the alloys more amenable to welding and thermo-mechanical processing.
Known γ″ alloys, for example, IN718 and IN725 cannot provide the balance of properties needed for operating at temperature in excess of 650° C. for prolonged periods. In particular, IN718 and IN725 are prone to microstructural instability. In the case of IN718, this is associated with the formation of the δ phase at the expense of the strengthening γ″ precipitates, leading to a loss of mechanical properties and unacceptably low component lives. In general, alloys of this type possess insufficient creep resistance, damage tolerance, environmental resistance and proof strength at temperatures in the range of 650° C. to 800° C. As such, they are not good candidates for service at peak temperatures above 650° C.
Accordingly, it is an aim of the present disclosure to provide an age-hardenable nickel-chromium alloy that possesses improved mechanical properties at high temperatures. It is also an aim of the invention to provide an alloy that may be used in conjunction with additive manufacturing and/or welding methods existing within the art.